No, I don't agree.
The best riders I've ever seen all have years, if not decades, of lessons, studying and mentoring to bring to them to that level.
And how do you define "best" - by competition results? You won't find very many self taught riders at the higher levels of competition. Training results? I think good trainers have to have input from a variety of different sources and even different disciplines, so I can't think of any really terrific entirely self taught trainers either.
Certainly I've seen people with natural talent, and learning core skills is easier for them than for folks without it, but I've seen both be types be successful, it's just easier for those with the gift of natural talent.
The one point of agreement we do have is this:
seen people who have taken lessons all their life and literally cannot ride properly to save their life
Yes, I've seen those people too, but my opinion is their problem is one of several things: 1.) anthropormorphization - ascribing human qualities to the horse, rather than learning how a horse thinks and reacts. People who think it's "mean" to discipline a horse, and bring them extra carrots so the horse will like them and take that canter lead; or people who are unwilling to be the boss 2.) poor instruction. Most typical is the negative method - students ride around in a circle and are told "Heels down!" "Don't slouch." "Don't get in his mouth!" but are never shown what they should be doing, given a positive model to emulate or have skills broken down into teachable steps. It's amazing that anyone ever learns to ride under this method and 3.) complete lack of fitness, athletic ability, timing or feel. Rare, but it does happen.